Method of handoff between base stations in a wireless communications system

ABSTRACT

A radio communications system ( 10 ) having a mobile station ( 30 ) and at least two base stations ( 13, 14 ). ATM radio channels ( 31, 32 ) are provided between the remote station and the base stations. Each of the ATM channels supports communication though ATM cells over a common frequency band. When handoff conditions are met for a handoff from the first base station to the second base station, a second virtual path identifier and a second virtual connection identifier are selected for a connection between the second base station and the remote station.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/615,381 filedon Mar. 14, 1996 now U.S. Pat. No. 5,940,381.

FIELD OF THE INVENTION

This invention relates to a wireless communications system utilizingasynchronous transfer mode (ATM communications) and it relates to remotestation (generically referred to as a ‘mobile station’ though notnecessarily mobile) for operation in such a system and, separately, abase station controller and a method of operation.

BACKGROUND OF THE INVENTION

In the modern telecommunications world, voice communications continue tobe a popular mode of communication, but new services like videotelephony, high speed data and short message services continue to expandon existing services. The arrival of new telecommunications servicesgenerates new requirements for telecommunications networks. Newtelecommunications techniques (transfer modes) are required and offerpossible advantages compared to existing techniques. Traditionaltransfer modes for wired communications are circuit switching, familiarin classical telephone services, and packet switching, familiar intelegraphy and modern short message service and data systems.

Asynchronous transfer mode (ATM) is a mode of fast packet switchingwhich allows systems to operate at a much higher rate than traditionalpacket switching systems. Features which characterize ATM communicationsare: the ability for asynchronous operations between a sender clock anda receiver clock; transmission “cells” of pre-defined sizes; andaddressing carried out in a fixed size header (that is not but time,frame position or other fixed characteristic). ATM communication issometimes also referred to as asynchronous time division (ATD)communications.

Asynchronous transfer mode (ATM) is a mode of fast packet switchingwhich facilitates switching systems that operate at a much higher ratethan traditional packet switching systems. Features which characterizeATM communications are: the ability for asynchronous operations betweena sender clock and a receiver clock; the concept of a “virtualconnection” which is established for the lifetime of an information flowthat comprises part or all of the communication; transmission “cells” ofa fixed, standardized size; and connection identification carried in afixed size header (that is not by time, frame position or other fixedcharacteristic). ATM communication is sometimes also referred to asasynchronous time division (ATD) communications. Other features of ATMcommunications are notions of a “service category”, “traffic contract”and Quality of Service objectives that apply to the virtual connection.The expression “virtual connection” here is used to refer a virtual pathand virtual circuit pair and “virtual connection identifier” meanseither a virtual path identifier (VPI) or a virtual circuit identifier(VCI) or both.

ATM communication has proven useful in high-value point-to-pointland-line communication, for example, satellite links and underseacables. ATM allows multiple simultaneous circuits, sometimes referred toas virtual circuits (VCs), to be established from end to end along thelink.

European Patent No. EP0679042 of Roke Manor Research describes a mobilecommunications network with ATM as the transfer mode used in theswitching infrastructure and describes steps to be taken in the mobilenetwork switching infrastructure when a mobile terminal changesaffiliation from one base station to another base station, as in aconventional handoff operation and when a mobile terminal communicatessimultaneously through more than one base station. The transfer mode ofthe radio link is not described. International Patent Application No.WO94/28645 of The Trustees of Columbia University in the City of NewYork also addresses the use of ATM in a mobile communications systemswitching network and addresses distributed call set-up and rerouting ina mobile ATM based system with ATM switches.

A mobile communications network consists of a number of mobile endsystems, a number of base stations, and a number of base stationcontrollers, where the base stations and base station controllers areinterconnected using an Asynchronous Transfer Mode (ATM) network. When amobile end system moves from radio site (or “cell” or “zone”) toanother, it is necessary to execute a handoff between the correspondingbase stations.

The standardized ATM architecture prohibits any ATM network (including awireless ATM network) from misordering or duplicating ATM user datacells. In general, ATM networks should lose (i.e. by discarding) few, orpreferably no, ATM user data cells at any time, including duringhandoff. Further, the ATM service architecture distinguishes between‘real time’ and ‘non-real time’ service categories. In real time servicecategories, cell delay variation (CDV—the variability in the pattern ofcell arrival events at the output of an ATM connection relative to thepattern of corresponding events observed at the input of the connection)is an element of quality of service. CDV is negotiated between the endsystems (including mobile end systems) and the network(s). If a cellexceeds the agreed CDV, then it either is lost, or becomes useless tothe end system when it is delivered; thus, a late cell is treated as ifit were lost. Non-real time services are indifferent to CDV, but may bemore sensitive to cell discard.

The arrangements described in the above prior art patent application arenot optimal in their use of ATM resources in an access network, nor dothose arrangements address communication using ATM as the transfer modeover-the-air.

International Patent Application No. WO94/32594 of NTT MobileCommunication Network, Inc. describes a cellular mobile radiocommunication system soft-handover scheme using code division multipleaccess (CDMA) where signals transmitted from different base stations arespread with different spread codes and simultaneously received at amobile station with reception units in correspondence to different basestations. It is described how communication can take place in packetswhich include a call number, in case the mobile station deals with aplurality of calls, a sequence number and an identification number (ID)for the mobile station. It is explained how the same packet can bereceived at the mobile station from more than one base station orreceived at more than one base station from the same mobile station, toprovide a reliable diversity handover scheme. The establishment ofsimultaneous communication through two base stations is described,without the completion of a handover process being described. It must beassumed that the completion of handover complies with pre-existing CDMAsoft handover principals. The patent application also mentions that thepacket communication scheme can be an ATM scheme.

Attention is turning to the use of ATM for the radio interface transfermode of wireless communications. There is, for example, a need forwireless users to have access to wired ATM networks and existing ATMsystems such as multi-media applications need a wireless platformproviding multi-media support. It is also recognized that systems suchas universal mobile telephone systems (UMTS) and existing wireless localarea networks (LANs) cannot meet all future data user needs. Efforts todate have been in the use of ATM in the wireless extension of fixedinfrastructure systems, such as local area networks (LANs) andintegrated service data network (ISDN).

For private land mobile networks and cellular radio networks,circuit-switched frequency-division multiple access (FDMA) with orwithout time division multiple access (TDMA), as well as code divisionmultiple access (CDMA) continue to be the available multiple accessschemes for the radio interface. Each of these multiple access schemeshas its advantages and disadvantages in different circumstances and thevarious schemes are generally incompatible with each other.

A mobile radio system is now envisaged using ATM as the transfer modeand using a novel multiple access scheme which has advantages overexisting FDMA, TDMA and CDMA multiple access schemes. There is a needfor a method of handover in such a novel system.

GLOSSARY OF TERMS

ATM Asynchronous Transfer Mode

BS Base station

BSC Base station controller

CDV Cell Delay Variation

CLP Cell Loss Priority

GFC Generic Flow Control

HEC Header Error Control

PTI Payload Type Identifier

VPI Virtual Path Identifier

VCI Virtual Circuit Identifier

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a mobile radio network.

FIGS. 2 through 6 are further block diagrams of the network of FIG. 1showing a sequence of connection configurations.

FIG. 7 is a ladder diagram showing an exchange of signaling messages fora base station to base station handoff.

FIG. 8 is a time sequence diagram showing the timing relationship ofcell streams during different handover scenarios.

FIG. 9 is a time sequence diagram illustrating a splicing operation in afirst scenario.

FIG. 10 is a time sequence diagram illustrating a splicing operation ina second scenario.

FIG. 11 is a block diagram of a BSC in accordance with one aspect of theinvention.

FIG. 12 is a bit map diagram of an ATM header with physical layerinformation added.

FIG. 13 is a block diagram of a mobile station.

FIG. 14 is a flow diagram illustrating operations performed by themobile station of FIG. 13.

FIG. 15 is a timing diagram illustrating power saving features of themobile station of FIG. 13.

FIG. 16 is a flow diagram showing further operations performed by themobile station of FIG. 13.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a radio communications system 10, which comprises a numberof base station controllers (BSCs) of which two are shown as BSCs 11 and12, each controlling a number of base stations (BSs) 13, 14, 15 and 16by way of example. Each BSCs, e.g. BSC 11 communicates with associatedbase stations 13 and 14, with one or more fixed ATM end systems 17 andwith other BSCs, e.g. BSC 12, by means of a fixed ATM network 20, whichis implemented using existing standards and implementation agreements.The fixed ATM network 20 is comprised of a mesh of interconnected ATMnodes, of which three (nodes 21, 22 and 23) are shown in FIG. 1 by wayof example.

Node 21 is connected to BSC 11 and BSs 13 and 14 and to node 22. Node 22is connected to BSC 12, via a first port 24 of BSC 12, and fixed to endsystem 17. Node 23 is coupled to BSC 12 via a second port 25 of BSC 12and is coupled to BSs 15 and 16. Thus different configurations arepossible and a node, e.g. node 21, can pass downstream traffic from thefixed end system 17 to the BSC 11 and from the BSC 11 to a BS 13 whilesimultaneously passing upstream traffic from a BS 13 to the BSC 11 andfrom the BSC 11 to the fixed end system 17. Alternatively, a BSC, e.g.BSC 12 can effectively divide the network into a sub-network on themobile side and a sub-network on the fixed side.

In a real system, there will be many more nodes in the network 20 andany given link between the fixed end system and a base station via a BSCmay pass through many nodes in sequence.

The network 10 further includes a number of mobile ATM end systems(alternatively “remote stations” and hereafter simply “mobilestations”), which communicate with the base stations by radio. One suchmobile station 30 is shown by way of example. An object of end-to-endcommunication is the establishment of one or more ATM virtualconnections between the mobile station 30 and one or more fixed endsystems 17. The mobile end system moves during the lifetime of the ATMconnection, moving from the radio cell covered by the first base stationto the radio cell covered by the second base station and so on. FirstATM radio channel 31 is provided between mobile station 30 and basestation 13 and second ATM radio channel 32 is provided between mobilestation 30 and base station 14. First and second ATM radio channels 31and 32 support communication though ATM cells over a common frequencyband and are described in more detail below. Base station 13 has aconnection identifier memory 35 and base station 14 has a connectionidentifier memory 36.

As the mobile end system travels from radio cell to radio cell in thedirection of the arrow, it is desirable that the connection segmentbetween the old base station and the BSC (for inter-base stationhandoffs), as well as the connection segment between the new anchor BSCand the old BSC (for inter-BSC handoffs) be removed from the connection,so as to release resources and minimize fixed and variable delay. To theextent possible, it is desirable to use existing ATM standards andimplementation agreements. In particular, mobile-specific modificationsto the fixed ATM network should be avoided.

There is a trade-off to be made with regard to the use of ATM virtualpaths (VPs). A VP is an aggregation of virtual circuits, which can beprocessed by an ATM VP switch as a group rather than individually. Ifall virtual circuit connections (VCCs) to a single mobile end system areaggregated into a single VP connection, handoffs can be executed on theaggregate, minimizing processing. However, the service category andquality-of-service provided for the VP are at least as stringent asthose for the most sensitive VCC that uses the virtual path connection(VPC), and the traffic contract for the VPC is at least as large as thesum of the traffic contracts for all of the possible VCCs (noting thatVCCs can be added after the VPC is established). This approach is moreefficient in this respect than arrangements in the prior art.

Although the preferred embodiment of the present invention uses switchedVCCs from the BSC to the BS, an alternative embodiment uses switchedVPCs (i.e. established on demand using signaling). In this case, thereis a VPC for each mobile device. The traffic contract is sufficient tohandle only existing VCCs and some additional margin; it can be adjustedas needed during a handoff or by means of signaling renegotiation.

FIGS. 2 through 6 represent a sequence of connection configurations inthe system of the preferred embodiment of the present invention, as themobile end system moves. The heavy lines in each figure show theconnection configuration.

A BSC, e.g. BSC 11 in FIG. 2, is the endpoint of a point-to-pointbidirectional virtual circuit connection (VCC) 40, where the otherendpoint of the VCC is the fixed end system 17. Thus, BSC 11 appears tothe ATM network 20 to be an end system. The BSC 11 is also an endpointof two or more virtual connections, which may be VCCs or virtual pathconnections (VPCs). One of these virtual connections, which isdesignated the downstream virtual connection 41, is a unidirectionalpoint-to-multipoint connection, having its root at the BSC 11, such thatATM cells flow from the BSC to all BSs that are presently joined to thedownstream virtual connection. The other virtual connection, which isdesignated the upstream virtual connection 42, is a unidirectionalpoint-to-point connection, such that ATM cells flow from a single basestation 13 to the BSC 11. Point-to-point and point-to-multipointunidirectional virtual connections and the means for signaling to obtainthem are defined in the ATM standards. The BSC 11 splits the twodirections of the connection between itself and the fixed end system byswitching cells received from the fixed end system 17 to the downstreamvirtual connection 41 and by further switching cells received on theupstream virtual connection 42 towards the fixed end system.

FIG. 2 shows the initial configuration of ATM connections. An end-to-endcommunication has been established by means known in the art, includingestablishment of the upstream and downstream virtual connections, bymeans of standardized ATM signaling protocols. The downstream virtualconnection 41 has only one leg, so is indistinguishable in the figurefrom a point-to-point connection. Thus, cells sent by the fixed ATM endsystem 17 are forwarded by the BSC 11 to the base station 13 serving themobile station 30, and cells sent by the mobile station 30 are forwardedby the base station 13 to the BSC 11, and thence to the fixed end system17.

FIG. 3 shows the configuration of the ATM connections during a BS—BShandoff from the old base station 13 to a new base station 14. The BSC11 has determined that the handoff is available or required. Thisdetermination may take one a number of forms. In the preferredembodiment the mobile station 30 reports signal strength and bit errorrate measurements to the BSC 11 indicating the need for a handoff.Either the mobile station 30 reports to the BSC 11 the detection ofsynchronization cells (F3 cells) transmitted by base station 14 andidentifying base station 14 or base station 14 reports to the BSC 11 thedetection and reception of ATM cells from the mobile station 30.

The BSC 11 uses existing ATM call control signaling protocols to add aleg 50 to the downstream connection, having as its destination the newbase station 14. It further uses the signaling protocols to establish anupstream connection 51 to the new base station 14. In the downstreamvirtual connection, the ATM network node 21 bifurcates the ATM cellstream into connections 41 and 50. This is achieved by the BSC 11providing the node 21 with ATM signaling requesting the bifurcation.

Thus, in FIG. 3, the mobile station 30 is receiving two copies of thecell stream that originated in the fixed end system 17, and the BSC 11is receiving two copies of the cell stream that originated in the mobilestation 30. Initially, the mobile station and the BSC each discard cellsreceived from the new base station 14, and continue to consume cellsreceived from the old base station 13. After synchronization (describedbelow with reference to FIGS. 8 and 9) is performed, the mobile endsystem and the BSC discard cells received from the old base station 13and consume cells received from the new base station 14.

FIG. 4 shows the configuration of the ATM connections after the BS—BShandoff. After it has completed synchronization, the BSC uses theexisting ATM call control protocol to release the old upstreamconnection 42. Similarly, after it has completed synchronization, themobile station 30 drops the old leg of the downstream connection 42.

FIG. 5 shows the configuration of the ATM connections for a BSC-to-BSChandoff as mobile station 30 moves from the coverage area of BS 14 tothe coverage area of base station 15 served by BSC 12. Mobile station 30reports through base station 14 that it is receiving cells from basestation 15 and optionally reports the signal strength and/or bit errorrate of those cells. The preferred operation is that mobile station 30passes these cells to BSC 11 and BSC 11 examines the error rate withinthe cells by performing error detection on the cells.

As a third alternative, base station 15 reports to BSC 12 via node 23the detection and reception of ATM cells from mobile station and BSCidentifies that BSC 11 is the BSC serving the mobile station and reportsto BSC 11 that there is an opportunity for a handoff. BSC 12 identifiesBSC 11 as the serving BSC either by interrogation of surrounding BSCs orby information reported to it periodically from surrounding BSCs as tothe identification numbers of mobile station being served by thesurrounding BSCs.

Node 21 (or some other node) is instructed by BSC 11 to bifurcate thedownstream connection 50 and establish a connection 60 to BSC 12, wherethe connection is made through first port 24. BSC 12 in turn establishesa connection 61 through its other port 25 to base station 15 via node23. BSC 12 then establishes an upstream connection 62 to BSC 11. BSC 11combines the upstream connection 62 with the upstream connection 51 in amanner described below.

Finally, as shown in FIG. 6, BSC 11 instructs node 21 to drop downstreamconnection 50 and mobile station 30 drops the upstream connection 51.

FIG. 7 is a ladder diagram showing the exchange of signaling messagesfor a BS-BS handoff as shown in FIGS. 2, 3 and 4. The messages indicatedby thin lines are standard ATM connection control signaling messages.The message 70 indicated by the dotted line is an additional messagessent between the BSC 11 and the mobile station 30. The vertical linesshow elements of the connection configuration. Messages 75 and 76 aresetup and connect messages for the downlink connection between the basestation 14 and the mobile station 30. Messages 77 and 78 are setup andconnect messages for the uplink connection between the base station 14and the mobile station 30

In the process of ATM signaling to add or remove legs from connections,there is a correlation identifier which is part of the signaling messagesent (by the mobile station or the BSC) to the network and transferredend-to-end. This identifier maps the VPI and VCI combinations betweenthe connection for the old base station with the VPI and VCI combinationfor the connection to the new base station. Each of the messagesindicated by the thin lines in FIG. 7 carries this correlationidentifier.

Reference is now made to the handoff process and the appropriate VPI andVCI selection in the downlink connection setup message 75 in FIG. 7 inwhich the base station 14 sends a setup message to the to the mobilestation 30. The VPI is generally unique for the connection between thetwo end points and is selected by the base station 14. The VCI can bethe same as for the cell stream from base station 13 to mobile station30.

Mobile station 30 receives from BSC 11 a correlator identifieridentifying that the new connection (having a new VPI and VCI) is thesame as the existing connection through the old base station. Mobilestation 30 is able to distinguish between the cell streams by virtue ofthe different VPI/PCI combinations. BSC 11 instructs mobile station 30to initiate a handover to the new virtual connection identified by thecorrelation identifier and the new VPI/VCI combination.

It is preferred that across the whole network 20 the whole domain of VPInumbers, at least for downlink connections, is subdivided into mutuallyexclusive sub-groups of VPIs (or VPIs and VCIs) and that any given basestation uses only its allocated sub-group of VPIs. These are stored inthe connection identifier memory 35 or 36 for the base station. Adjacentbase stations are, as far as practicable, not allocated the samesubgroup of VPIs. This has an advantage similar to the reuse offrequencies in an FDMA system in that the VPI sub-groups are re-usedacross the network and confusion is avoided at the overlapping regionsof base stations or the overlapping regions of networks. In one of thefinal steps of the handoff process, the VPI number may be changed to anew VPI number selected by the new base station. In this embodiment theVPI number is temporarily out of the sub-group of VPI numbers allocatedto the base station and is chosen from within that sub-group when it isupdated.

In operation, an ATM communication connection is established between themobile station 30 and the old base station 13 with at least a firstvirtual connection identifier (preferably a VPI) selected from theconnection identifier memory 35. When it is determined that handoffconditions are met for a handoff to the new base station 14, basestation 14 selects a second virtual connection identifier (including asecond virtual path identifier and a second virtual circuit identifier)for a downlink connection between the new base station and the mobilestation. In the preferred method, the new base station 14 at leasttemporarily selects the existing virtual path identifier and theexisting virtual circuit identifier as the new virtual path identifierand the new virtual circuit identifier for uplink communication. Atleast one of the second virtual path identifier and the second virtualcircuit identifier (preferably the former) is later changed to a newvalue.

Thus the first base station 13 is provided with a first sub-group ofvirtual path identifiers in memory 35 for use in communications with themobile station 30 and the second base station 14 is provided with asecond sub-group of virtual path identifiers in memory 36 for use incommunications with the mobile station 30, which is mutually exclusiveto the first sub-group of virtual path identifiers, and the secondvirtual path identifier is (later, if not initially) selected by thebase station from the second sub-group.

Each base station communicates with its mobile stations through ATMcells and each base station transmits physical layer synchronizationcells using its own synchronization timing. ITU Rec. 1.610 describesvarious types of cells including F3 cells and F5 cells. Synchronizationtakes place in the physical layer using F3 cells and in the ATM layerusing F5 cells.

In the physical layer, the physical radio channel is divided intoframes. One frame comprises a fixed number of cells, there beingpreferably more than 10 and less than 50 cells per frame. Each Nth cellis a synchronization cell (where 10<N<50) which can be considered to bean F3 cell. Thus the frames received by the mobile station 30 from basestation 13 may be offset from the frames received from base station 14.The offset is not necessarily a whole number of cells, but is entirelyarbitrary. This is advantageous from a cell planning point of view.Operation takes place at a basic frame rate, with all transmissionsbeing at integer multiples or devisors of the frame rate. A virtualconnection comprises one or a plurality of cells per frame, depending onthe desired data rate and the available capacity.

Each cell has a header (described below with reference to FIG. 12)identifying the VPI and the VCI. Cells with the same VPI and VCI arecollected by the base station (in the uplink direction) or the mobilestation (in the downlink direction) into blocks of cells. The first cellof each block is an ATM synchronization cell, which can be considered tobe an F5 cell.

FIG. 8 is a time sequence diagram, showing the timing relationship forblocks of cells between the cell stream 100 from the old base station 13and the cell stream 102 from the new base station 14 in scenarios wherethe fixed delay from the new base station is approximately the same asthe fixed delay from the old base station. The figure also shows analternative cell stream 104 from the new base station where the fixeddelay from the new base station is greater than the fixed delay from theold base station and a further alternative cell stream 106 where thefixed delay from the new base station is less than the fixed delay fromthe old base station.

Synchronization cells 110, 111 and 112 are present in the cell streamfrom the old base station 13 at intervals of fixed numbers of cells,where the interval is known to the base station, mobile end systems andBSC. Synchronization cells 120, 121 and 122 are present in the cellstream from the new base station 13 at the same intervals. (In cellstream 106 a further synchronization cell 123 is shown.) In each case,the differential fixed delay is approximately equal to the difference inarrival times between a synchronization cell from the old BS and thecorresponding cell from the new BS. Due to the effect of delay jitterand queuing delays, the exact difference is not known. In each case asplicing arrangement is needed to seamlessly end the cell stream 100from the old base station and pick up the cell stream 102, 104 or 106from the new base station without omitting or repeating cells.

FIG. 9 is a time sequence diagram, showing the operation of the splicingprocess carried out in the mobile end system, where it acts upon the twolegs 41 and 50 of the downstream virtual connections that arrive fromthe old base station and the new base station 13 and 14, respectively.The splicing process also occurs in the base station controller 11 (the‘anchor’ BSC), where it acts upon the two upstream virtual connections42 and 51 that arrive from the old base station and the new basestation, respectively.

FIG. 9 shows the cell streams 100 received from the old base station,the cell stream 102 received from the new base station, and the cellstream 200 emerging from the splicing process, respectively. Forillustration, cells received from the new base station are in phantomoutline. User data cells 130, 131 etc. are designated sequentially(i.e., n, n+1, n+2, etc.), and synchronization cells 110 are designatedsequentially (i.e., sync_(m), sync_(m+1), etc.), where any cell sodesignated is identical whether received from the old base station orthe new base station. At the beginning of the splicing process, userdata cells n, n+1, n+2, n+3 received from the old base station becomethe output of the splicing process. The splicing process awaitssynchronization cells. If synchronization cell 110 (sync_(m)) isreceived first from the old BS, then the splicing process discardssubsequent user data cells 134, 135, 136 etc. (labeled n+4, n+5, etc)from the old BS, awaits the corresponding synchronization cell 120(sync_(m)) from the new base station, discards the synchronization cell120, and then the output of the synchronization process becomes userdata cells 154, 155, 156, 157 etc. (labeled n+4, n+5, etc.), receivedfrom the new BS.

Referring to FIG. 10, if synchronization cell 120 (sync_(m)) is receivedfirst from cell stream 102 from the new BS 14, then the splicing processstores, in a first-in-first-out (FIFO) fashion, user data cells 154, 155and 156 (n+4, n+5 and n+6), until the corresponding synchronization cell110 (sync_(m)) is received from the old BS; at that time, the storeduser data cells 154, 155, 156 are removed from the FIFO storage, andbecome the output of the splicing process; when the FIFO storage becomesempty, then subsequent user data cells 157 received from the new BSbecome the output of the splicing process.

Cells may need to be removed from the FIFO storage at a rate which ispaced by the peak cell rate, sustainable cell rate or available cellrate.

Referring now to FIG. 11, details of a BSC 11 (or 12) are shown. The BSCcomprises an ATM switch 300 having input port 301 arranged to receivevirtual connections 42, 51 and 62 (and the downlink part of connection40) from node 21 of the ATM network 20 (these virtual circuits beingbundled by node 21 over the same virtual path). It has combiner 303coupled to switch 300 and an output port 304 for coupling to node 21 (orto some other node in the sub-network on the fixed side). Combiner 303comprises buffers 306 and 307, splicing element 308 and processor 309.The BSC 11 also has an input port 320 for coupling to node 21 (or tosome other node in the fixed end system subnetwork) coupled to an outputport 321 for coupling to node 21 or some other node in the sub-networkon the mobile side. In addition it has ATM signaling circuit 330 havingan output 331 coupled to output port 321 ad it has control processingelement 332 coupled to the combiner 303 and the ATM signaling circuit330 for control of those elements.

In operation, the scenario will be considered where a BS-to-BS handoveris in progress at the stage shown in FIG. 3. Virtual connection 42 frombase station 13 and virtual connection 51 from base station 14 arereceived on port 301 (together with the downlink part of connection 40which need not be considered). ATM cells of connections 40 and 51 arepresented at port 301 with the same virtual path numbers. ATM switch 300separates these cell streams by their different VCIs and passes them tobuffers 306 and 307. One of buffers 306 and 307 acts as a FIFO to bufferup cells arriving from the new base station (over connection 51) whenthe synchronization cell 120 arrives from connection 51 before thesynchronization cell 110 from connection 40. Processor 309 removes thesynchronization cells 110 and 120 and performs the other operations ofthe splicing process described above, including the control of the rateof removal of the cells from the buffers 306 and 307.

For the downlink direction, ATM signaling circuit 330 issues ATMcommands 334 and inserts these into the downlink connection to the ATMnetwork node 21. These messages include messages to: (a) establish newconnections; (b) add new legs to existing connections; (c) remove legsfrom existing connections and (d) drop connections. Thus ATM signalingcircuit 330 issues an instruction to node 21 to add leg 50 to existingconnection 41.

Thus it has been described how existing point-to-multipoint andpoint-to-point unidirectional ATM connection configurations are used ina novel way, along with standardized connection control signalingprocedures, to transport a bifurcated ATM cell stream during a handoff.Existing, standardized, operations and maintenance (OAM) cell formatsand procedures are extended to synchronize the handoff such thatduplication and misordering are prevented, and loss is avoided. Forreal-time service categories, the synchronization procedures providecompensation of differential delay between the old path of the virtualconnection and the old path of the virtual connection, so that CDVobjectives can be met.

The arrangement has the advantages that: cells are not duplicated ormisordered during handoff; for non-real time services, cells are notdiscarded if buffers are dimensioned properly; for real time services,CDV objectives are met, or cells are discarded; further, if sufficientlyconservative CDV objectives are set, cell discard does not occur; thepath of a connection follows a spanning tree from the anchor basestation to the mobile end system; thus, the number of connectionsegments (and the corresponding resources) is minimal; standardized ATMlayer and ATM signaling protocols are built upon but not modified.

In this manner, combining of virtual circuits on the uplink andbifurcating of virtual circuits on the downlink is achieved.

The above description has set out the elements of the networkinfrastructure and their operation. The features of the mobile station30 and the novel air interface between the mobile station 30 and itsbase station are now described.

It has been described that in the radio interface physical layer theradio channel is divided into frames, each frame comprising a fixednumber of cells and each Nth cell being a synchronization cell (where10<N<50). FIG. 12 shows a bit map for the header of a cell, whether thisis a data cell or a synchronization cell. ATM cell header part 400comprises 5 octets. Four bits are for generic flow control, eight bitsare for VPI, 16 bits are for VCI, 3 bits are for payload typeidentifier, one bit is for cell loss priority and one octet is forheader error control. It can be seen that the VPI and the VCI are afixed resource. There is a need to make efficient use of this resource.The PTI field identifies, among other things, whether the cell is asynchronization cell or some other cell type.

Added to the ATM cell header part 400 is a physical layer part 401.Physical layer part 401 is shown as comprising only one octet, but maybe longer. For present purposes, it is illustrated as having sufficientspace for a cell sequence number of 8 bits.

The header shown in FIG. 12 accompanies a payload of 48 octets. This isfixed in the ATM network but may have a trailer added in the physicallayer, for example giving extra cyclical redundancy checking or othererror control code.

As an alternative arrangement to that illustrated in FIG. 12, physicallayer header 401 is omitted and a cell sequence number is inserted inthe ATM header 400 in place of some of the fields shown. For example,the GFC field can be omitted and the four bits of this field togetherwith four bits of the VPI field (or four bits of the VCI field) can beused as a sequence number field. The sequence number field is preferablylarge enough to span several blocks of cells. If, for example, the blocksize is 64 cells, an 8-bit sequence number field spans 4 blocks beforeit has to repeat. By providing a block sequence number in each block,these two numbers together uniquely identify a cell over a very largenumber of cells.

Referring to FIG. 13, elements of an example of a mobile station 30 inaccordance with an aspect of the present invention are shown. The mobilestation comprises a transmitter 501 and a receiver 502 coupled to anantenna switch 503 and, through the antenna switch, to an antenna 504. Asynthesizer 505 is coupled to each of the receiver 502 and thetransmitter 501. A demodulator 510 is coupled to receiver 502. Amodulator 511 is coupled to the synthesizer 505. A logic unit 520 iscoupled via data lines 521 and 522 to the demodulator 510 and modulator511, respectively, and is coupled by control lines 523 and 524 to thedemodulator 510 and the receiver 502 and to the transmitter 501 and theantenna switch 503 respectively. A control bus 526 is coupled betweenthe logic unit 520 and the synthesizer 505. Synthesizer 505 and controlbus 526 are optional, as it is not necessary for the mobile station toperform FDMA channel changing, nor is it necessary to perform CDMAspreading and de-spreading. Instead of an antenna switch 103, a duplexercan be used, allowing simultaneous receiving and transmitting of ATMcells. Logic unit 520 has an associated FIFO buffer 540.

Coupled to the logic unit 520 via a digital bus 528 is a processor 530.Coupled to the processor 530 is a random access memory (RAM) 531, aprogram memory in the form of electrically erasable programmableread-only memory (EPROM) 532, an operator interface 533 such as akeyboard and display and an I/O interface 535.

In operation, the logic unit 520 receives data for transmission from theprocessor 530 and generates ATM cells. The ATM cells are created byassigning an ATM header to each cell comprising a virtual pathidentifier and virtual circuit identifier for the particulartransmission. Logic unit 520 adds a physical layer header (and trailerif required) providing a sequence number for each sequential cell andsupplies the resultant transmission burst data to modulator 511. Itwill, of course, be appreciated that alternative arrangements can beprovided. For example the addition of the physical layer header andtrailer, can be carried out in processor 530.

The logic unit 520 passes the transmission burst data to the modulator511 bit-by-bit and provides a transmitter key-up signal on control line524 (at the same time switching antenna switch 503 to the lower positionas shown). The logic unit 520 controls the timing of key-up of thetransmitter 501, so that each transmission burst is transmitted at acarefully selected time in a frame.

When the transmitter 501 is not keyed up for transmission, the controlline 524 causes the antenna switch 503 to switch to the upper positionas shown, allowing ATM cells (with physical layer header and trailer) tobe received via the antenna 504 to the receiver 502 and demodulated bythe demodulator 510 and passed to the logic unit 520.

The received ATM cells are identified in the logic unit 520 by thevirtual path and virtual circuit identifier in the header 400 and onlycells received with the appropriate virtual path and virtual circuitidentifier are selected by the logic unit 520 for further processing.Logic unit 520 orders the received ATM cells in the correct order asdefined by the sequence numbers in the physical layer headers 401. Logicunit 520 also performs error correcting in a manner known in the art.When the data has been corrected, the data is passed on to the processorand to the upper layers of the protocol.

The processor 530 can perform the operation of assembling and orderingthe ATM cells and can perform the error correcting if desired, but thesefunctions can generally be performed more quickly in the logic unit 520.

Logic unit 520 provides wake-up signals over control line 523 toreceiver 502 (and to other parts of the mobile station) causing receiver502 to power up and power down. Powering up and down of a receiver inresponse to a signal is readily understood by one skilled in the art anddetails such as an electronic switch and a battery power source need notbe described here.

Logic unit 520 also controls synthesizer 505 via control bus 526 toselect appropriate frequencies for transmission and reception dependingon the particular frequencies of the system and the modulation schemeand other aspects of the physical layer.

FIG. 14 is a timing diagram for the purposes of illustrating operationof the mobile station 30 of FIG. 13. In the upper part of the diagramthere is a cell stream 700 which is the activity in real time on thedownlink of the first ATM radio channel 31. The cell stream 700comprises a number of transmission bursts 701, 702 etc., each burstcomprising one ATM cell with its radio interface header and trailer. Forthe purposes of illustration, the first burst 701 shown comprises asynchronization cell S1. This is a physical layer synchronization cell,distinct from synchronization cells 110 and 120 of FIGS. 9 and 10 whichare ATM layer synchronization cells. This burst 701 and latersynchronization cell burst 710 are separated by one frame of N ATM cellbursts 702, 703 etc. (the diagram is not to scale, as there is adiscontinuity shown between ATM cell burst 706 and synchronization cellburst 710). In the example illustrated, bursts 706 and 715 contain cellshaving the same VPI and VCI (connection A) and bursts 703, 704, 712 and713 contain cells having another VPI and VCI (connection B). One ofthese cells may be ATM layer synchronization cell 110.

Below cell stream 700 is illustrated cell stream 720. Cell stream. 720is the activity in real time on the downlink of the second ATM radiochannel 32 and comprises physical layer synchronization cell bursts 721and 730 marking the frames on the physical channel. These are separatedby the same frame length (N cells). Bursts 726 and 735 show anotherindependent connection on the channel (connection C). Bursts 725 and 734show that the ATM cells of connection A are being received on thisphysical channel. Note that the frequency and bandwidth of this channelare the same as the frequency and bandwidth of the physical channelsupporting cell stream 700. Note also that there is not necessarily anycode-divided spreading of the different physical channels. The twochannel are able to co-exist by virtue of careful selection by each basestation-mobile station pair of time slots that are available for thatpair.

Thus, for example, connection C in cell stream 720 is established duringgaps in the cell stream 700. Connection A over cell stream 700 is alsoestablished during gaps in cell stream 700. Synchronization cell bursts721, 730 in cell stream 720 are shown as coinciding with cell bursts703, 704 etc. because it is wholly possible that bursts 721, 730 etc. donot interfere with the mobile station communicating over bursts 703, 704etc. by virtue of the location of that mobile station and its powerselection.

Time lines 740 and 750 show wake-up times for mobile station 30. Beforethe handoff, logic unit 520 of mobile station 30 is powering up itsreceiver 502 over control line 523 during time periods T1, T2, T3 andT4—that is to say only at times coinciding with bursts in the framerelevant to the mobile station 30 (in particular ATM cell bursts for theconnection supported and synchronization cell bursts for the physicalchannel).

When a handoff conditions are met, i.e. handoff is perceived asavailable or a command is received from the communicating base stationrequiring a handoff, the logic unit 520 of the mobile station 30 powersthe receiver 502 up by providing a signal over control line 523 for alonger time period T5 sufficient to encompass the arrival ofsynchronization cell burst 721 of cell stream 720 and ATM cell burst 725of the connection supported. Thereafter it can power down until the nextfollowing synchronization cell burst 730 of the new base station cellstream 720. Other arrangements can be envisaged where the mobile stationextends its receiver wake-up time during the handoff and reduces it whensynchronization of the cell streams is complete. At a minimum, it mustremain in receive mode until synchronization cell burst 721 from the newbase station is received.

Thus a method of operation of the mobile station has been describedcomprising the steps of powering up the receiver 502 during first timeperiods T1 corresponding to physical layer synchronization cells 701arriving from the old base station and second time periods T2corresponding to ATM cells arriving from the old base station,determining that handoff conditions are met and powering up the receiverfor a third time period T5 longer than the first and second timeperiods. The third time period extends at least until a physical layersynchronization cell 721 is received from the new base station andpreferably at least until an ATM cell 725 is received from the new basestation following the physical layer synchronization cell from the newbase station. After a handoff from the first base station to the secondbase station, the receiver is powered up during fourth time periods (T6or T7) shorter than the third time period (T5).

Time line 760 illustrates operation of the mobile station 30 intransmission, i.e. the uplink cell stream over the R.F. interface. Intransmission, during the handoff, the mobile station simultaneouslytransmits its uplink cells over the virtual connection to base station13 and the virtual connection to base station 14. This is achieved inone of two ways.

The first and preferred way is illustrated in FIG. 14 and shows that acell 761 containing uplink data (or an uplink F5 synchronization cell)is transmitted and after a full frame period, the next cell 762 of thesequence is transmitted. These are marked as connection A′ and form theconnection to the old base station. As soon as possible after cell 761,the same cell is transmitted but with the VPI and VCI appropriate to theconnection to the new base station. This is shown as cell 771, and aframe later the next subsequent cell 772 is transmitted. Thus there isduplication of the transmission of the cell payload, with differentheaders. Note that the locations of cells 761 and 771 are selectedaccording to the activity on the uplink channel (which preferably has adifferent frequency band to the frequency band of the downlink channelbut could indeed share the same frequency band). Note also that thetiming of transmission of the uplink cells is selected so as not tocoincide with the corresponding cells on the two downlink channelsrepresented by cell streams 700 and 720. This is advantageous forantenna switching and receiver sensitivity reasons.

The second way of simultaneously transmitting uplink cells over thevirtual connections to the two base stations is by selecting the VCI andVPI for the new uplink connection as being the same as the VPI and VCIfor the existing uplink connection and transmitting each cell only once.In this scheme, commands 77 and 78 of FIG. 7 do not require theestablishment of a new connection, but command 77 merely informs themobile station of the acceptance of the cells by the base station andcommand 78 is an acknowledgment. As one of the final steps of thishandoff process, the VPI number can be changed to a new VPI numberselected by the new base station.

FIG. 15 is a flow diagram illustrating a splicing process performed bythe logic unit 520 of the mobile station 30 of FIG. 13. In step 800, thestep in the handoff process has been reached at which duplicate cellsare arriving at the mobile station 30 relating to the samecommunication, but arriving from different base stations in a manner thesame as has been described with reference to the cell streams arrivingat the BSC as illustrated in FIG. 8. Received physical layersynchronization cells are discarded. In step 801, cells from the newbase station are stored in FIFO buffer 540. When handoff conditions aremet (step 802), the mobile station 30 waits for the next (or first) ATMsynchronization cell (similar to cell 120 in FIG. 8 but this timereceived over the air within one of cell bursts 725, 734 etc.) from thenew base station (step 803). If in step 804 the ATM synchronization cellfirst arrives from the old base station (as in FIG. 9), step 805discards subsequent user data cells from the old base station andoutputs to the higher layers of the protocol the user data cells fromthe new base station after synchronization with the new base station.Otherwise (step 810) user data cells from the new base station arestored in FIFO buffer 540 until the next ATM synchronization cell(similar to cell 110 in FIG. 8) is received from the old base station,in a manner similar to that shown in FIG. 10. Eventually (step 812) theATM synchronization cells are discarded and the resulting continuousspliced cell stream is passed to the upper layers of the protocol andeventually to an application layer where the data is output to the userthrough operator interface 533 as voice or message text or video or inwhatever form the application dictates or it is passed on to some otherdevice over interface 535.

The splicing process can be modified to include synchronization so thatit supports real-time service categories. Each node in the ATM virtualconnection, the base station and the BSC(s) are required in the standardATM architecture to determine the maximum CDV that it will insert in theconnection (its CDV allocation). This requirement is extended to alsorequire a CDV allocation for the synchronization process, to be includedin the CDV allocation of the BSC or mobile station in which thesynchronization process resides. Further, the standardized ATMconnection control architecture allows the BSC to determine the largestcumulative CDV that could be inserted by those nodes upstream of itself.

FIG. 16 and the following description describes further details of thesplicing process performed in the logic unit 520 (or the processor 530)in the mobile station 30. Similar processing is performed in the BSC 11(or 12).

For real time service categories, the logic unit 520 and the processor530 between them control the rate R at which ATM cells are transferredacross the interface 535. The rate R is ordinarily determined atconnection establishment time using information contained in thestandard connection control signaling messages. If ATM cells arrive atthe logic unit at a rate faster than R, then the cells are stored inFIFO 540 (or in RAM 531) until they are able to be consumed at theinterface 535.

Referring to FIG. 16, the approximate differential delay D between thecommunication from the old BS and the communication from the new BS ismeasured in step 900 by determining the time between the arrival of async cell sync_(m−1) from the old BS and the corresponding arrival ofsync_(m−1) from the new BS. If (step 902) the absolute value of D isless than a predetermined delay value, then the splicing processproceeds in step 904 as shown in FIG. 9 or FIG. 10. No further delaycompensation is needed.

If step 906 determines that synchronization cell synchm−1 from the oldbase station arrives before synchronization cell synchm−1 arrives fromthe new base station, then in step 908 the synchronization processcalculates a compensating FIFO depth F for FIFO buffer 540, such that Fis a number of cells that need to be stored to allow the handover totake place without exceeding the CDV allocation for the synchronizationprocess. Step 910 then calculates a rate reduction factor r<1, such thatR*r allows F cells to accumulate in the FIFO in k blocks. The value of ris further determined such that reducing the service rate in step 912 atinterface 535 to R*r will not cause the CDV allocation to be exceeded.Except when D is exceptionally large, r will be equal to 1. When theFIFO contains F cells (step 914), then splicing can proceed asillustrated in FIG. 9 (step 916). At the same time (step 918), theservice rate at interface 535 is increased to R and the splicing processis concluded (step 920).

If, in step 906, synchronization cell synchm−1 from the old base stationarrives after synchronization cell synchm−1 arrives from the new basestation, then synchronization is performed as illustrated in FIG. 10(step 930). The clustered cells received from the new base station,cells 154, 155, 156, are stored in the FIFO 540. A rate increase factorr′ is calculated in step 932 such that increasing the service rate atthe interface 535 to r′*R will not cause the CDV allocation for thesynchronization process to be exceeded. The service rate at theinterface 535 is then increased in step 934 to r′*R. When the FIFObecomes empty (or nearly empty) as determined by step 936, the servicerate at interface 535 is reduced again in step 938 to R and the splicingprocess is concluded (step 940).

Thus a handoff process has been described which comprises combiningfirst and second cell streams in a remote station (mobile station) of aradio communications system, comprising: receiving a first cell stream700 from a first base station 13, the first cell stream including firstsynchronization cells (preferably ATM synchronization cells e.g. cell110, but alternatively physical layer synchronization cells e.g. cell701); receiving a second cell stream 720 from a second base station 14,the second cell stream including second synchronization cells(preferably ATM synchronization cells e.g. cell 120, but alternativelyphysical layer synchronization cells e.g. cell 721); outputting thefirst cell stream until a first synchronization cell is received;receiving a second synchronization cell; and outputting the second cellstream following the second synchronization cell.

The handoff process has significant advantages over soft handoffprocesses in prior art systems such as CDMA systems, especially wheredata is conveyed, because it does not rely on correlation of the datacontent (e.g. voice correlation) but allows seamless splicing of the ATMcells carrying the data and avoids or minimizes data loss orduplication. It also has advantages in systems carrying data whereservice rate is important, such as video data, as it allows for smoothcontinuous flow control of the data without jitter.

Modifications of the arrangements described can be made within the scopeof the invention. For example it has been described how sequence numbersare provided for individual cells and how sequence numbers are providedin ATM synchronization cells (F5 cells). As an alternative one or otherof these sequence numbers can be omitted. Also it has been described howsynchronization cell bursts (F3 cells) are provided in the physicallayer and ATM synchronization cells (F5 cells) are provided in the ATMlayer. In an alternative embodiment only one of these sets ofsynchronization cells are used.

What is claimed is:
 1. A method of operation of a radio communications system comprising a remote station, a first base station and at least a second base station, having an ATM communication connection established between the remote station and the first base station with at least a first virtual connection identifier, the method comprising the steps of: determining that handoff conditions are met for a handoff to the second base station; selecting a second virtual path identifier and a second virtual connection identifier for a connection between the second base station and the remote station; and wherein the first base station is provided with a sub-group of virtual path identifiers for use in communications with the remote station and the second base station is provided with a second sub-group of virtual path identifiers for use in communications with the remote station, which is mutually exclusive to the first sub-group of virtual path identifiers, and the second virtual path identifier is selected from the second sub-group.
 2. The method of claim 1, wherein the step of selecting the second virtual connection identifier comprises selecting at least temporarily the first virtual connection identifier as the second virtual connection identifier, at least for ATM communications from the remote station to the second base station.
 3. The method of claim 2 comprising changing at least one of a virtual path identifier and a corresponding virtual circuit identifier at least for ATM communications from the remote station to the second base station to a new value.
 4. The method of claim 3, wherein the virtual path identifier is changed and the corresponding virtual circuit identifier is not changed. 